The Fab fragments of several monoclonal antibodies that bind digoxin and other cardiac glycosides have been screened for crystallization conditions. We have crystallized two of these in forms suitable for X-ray analysis. The anti-digoxin antibody 40-50 in complex with ouabain crystallizes with symmetry consistent with space group C2, with a = 92.6, b = 85.0, c = 73.0 A and beta = 131.7 degrees. This crystal form shows considerable non-isomorphism between crystals. A second anti-digoxin antibody, 49-10, crystallizes with symmetry consistent with space group P2(1)2(1)2, with a = 95.3, b = 147.1 and c = 76.2 A.